Photoelectric tube



, Se pt.22, 1936 E Ag 2,055,181

PHOTOELECTRIC TUBE Filed Nov. 26. 1930 FIG.

FIG. 2

ATTORNE V Patented Sept. 22, 1936 UNITED STATES 2,055,181. rno'roumzo'rmc TUBE Ernest Rostas, Paris, France, assignor to Western Electric Company, Incorporated, New York,

N. Y., a corporation of New York Application November 26, 1930, SerialNo. 498,290

In France January 6, 1939 9 Claims. 1 (01. 250 -215) of the bulb. Reverse electrolysisis then started This invention relates to the manufacture of photoelectric tubes v and more particularly to method for producing a light sensitive surface for such tubes- 1 r Y n An object of the invention is toeffect improvement in the method of supplying photoelectric tubes with an interior coating of photoelectrically active material. v v A further object of the invention is to effect improvement in the method of activating the photoelectrically active material. Alkali metals such as sodium and potassium are the metals usually employed because these are found to be extremely sensitive to light, but other alkali metals, such as lithium and caesium, sometimes are employed. V l

Various methodsare in common use for introducing alkali metals in a pure state into the interior of the glass or like envelopeon theinner surface of which the metal is to be deposited as a film to form the-cathode. l

One-such method of introducing potassium is that of repeated distillation of the potassium in vacuum. Such a method involvesthe use of expensive apparatus, consequently tubes produced by this method are quite costly.

a A feature of the present invention is concerned with an improved method for'impregnating the inner surface of the wall of a'glass envelope with ions of an alkali metal, such as potassium, by effecting electrolysis between a quantity of molten alkali metal salt contained in the envelope and aquantity ofthe same salt forming an external bath in which the envelope is partially immersed. 4 In an example of practice of the invention a quantity of molten potassium nitrite is introduced into a glass bulb which is partially immersed in a bathof the same molten salt. Contact is made to each lot of the molten alkali metal salt and an electriccurrent from a suitable source is passed therebetween and through the glass so that electrolysistakes place in the ordinary way and in such a direction that metallic ions from the salt v inside the bulb pass into the glass. By this means the sodium or like ions contained in the glass close to the interiorsurface are replaced by thepotassium ions from the molten salt inside the bulb. The bulb is then washed and cleaned thoroughly. An internal filament is now introduced, in the turns of which is placed a small piece of magnesium. The bulb is again highly evacuated and the filament heated to the vaporization temperature of the magnesium which is liberated in the form of a vapor and deposits on the cool portion by applying a suitable potentialbetween the heated filament and an external bath of molten salt with the result that the potassium ions passed by the first electrolysis into the glass wall from the molten alkali salt, are liberated into the bulb and deposited on' the magnesium. This potassium metal is condensed in the form of a light-sensitive layer on the wallsiof the tube everywhere except on the small areawhose outside surface is exposed to the molten potassium salt. This clear portion of the tube wall serves as a window for admission of light during the operation of the tube. A small shield prevents deposition of the magnesium, and potassiummetals in such a place as tocause electrical leakage. r One method for activating the light sensitive layer formed as describedabove, is eifected by introducing a very low pressure of hydrogen of the order of .001 millimeter of mercury and lighting the filament previously introduced into the bulb, in the presence of this gas. Hydrides of the metal comprising the" sensitive layer are formed and the activation iscompleted without anyglow discharge.

' For a m ore method and apparatusinvol'ved in this invention attention is called to the accompanying drawing and following description.

Fig. 1 is a front cross-sectional view of appacomplete, understanding of the ratus for effecting impregnation of the'inner walls of a tube container with an alkali metalby electrolysis. V v t Fig. 2 is a front cross-sectional view of apparatus for effecting release of such metal by reverse v electrolysis.

According to one feature of the present invention a glass bulb into which it is desired to introduce an alkali metal is placed in a bath of molten alkali metal salt there being also some molten alkali metal salt of the same character inside the bulb. Contact is made to eachlot of molten alkali metal salt and an electric current is passed therebetween and through the glass so that electrolysis takes place in the ordinary way and in such a. i

direction that metallic ions from the salt inside vthe bulb pass into the glass. By this means, the sodium or like contained in the glass close to the interior surface is replaced by the metallic ions from the molten salt insidethe bulb. The bulb is then washed, cleaned and exhausted by pumping, an internal filament being introduced. Reverse electrolysis is thenstarted with the result that the metallic ions of the alkali metal, passed by the .fii5t electrolysis into the glass from the molten,

pro

6 of the same character, within the container 5.

The heat necessary to keep the potassium nitrite in a molten state is supplied from a Bunsen burner I supported in any suitable manner. The glass container 5 has an exhaust-tube I6,"a hydrogen inlet tube 24 and a metal rod 9 which makes contact with the molten salt 6 on one end while the other end is connected by a wire to the positive terminal of a battery II), the negative terminal of the battery being connected through switch I I to the metalcontainer Iby. the binding post I2.

Fig. 2 shows. apparatus used for producing reverse electrolysis, in accordance with this 'inven tion, and is the same as Fig. l with a few exceptions, The same reference. numerals have been used in the .two'figures to indicate identical parts, In this figure thecontact rod 9 of'Fig. 1 hasbeenreplaeed'by a filament PI, and a shield I9, whosepurpose will be described later, is attached to'fthe filament leads at the base of the neck of the glass'vessel 5'. A battery 20 and a variable 'rfesistance' 2l are connected in series with the filamentll'l. An additional battery 22 is included in acircuit'between the filament I1 and theexterior molten potassium nitrite 4, contact with the latter being made through the metal containerl, by binding post I2. j i

' Accordingto a preferred example of the process aglass 'envelope'5 is partially immersed in a bath 7 of molten potassium nitrite 4 and a quantity 'of I f' l ng. velope molten potassium nitrite 6 is introduced inside the envelope '5 and by an electrolysis method potassium is caused to pass into the glass on the interior surface'oi the envelope 5. :An internal filament I1 is now'introduced in theturns of which is twisted a small strip of magnesium I8. The envelope is 'highly evacuated through tube I6 and isplaced in a bath of molten potassium nitrite 4; the filameritll is then'heat'ed and the magnesiumIB, is deposited'on'the cool portion of the walliof the bulb '5,a'small shield I9 is used toprevent the magnesium depositing in such position as to cause-an electrical leak. A potential from battery 22 is then appliedbetween the filament I1 and the molten nitrite '4 outside the envelope 5 byclosing switch 23 and potassium contained in the glass is deposited on the-magnesium. isrever'se: electrolysis is not carried on too only pota'ssium' is introduced into the en- 5, but'if the actionis carried on longer,

sodium' in -the glass also begins to appear and jan alloy of potassium and sodium can be prod'ucedin the envelopei The potassium tubes thus made can 'be'actiyated with'hydrogen-in a manner hereinafter described and filled witha gas such'as argon.

The magnesium coating onto which the potas siumis, deposited maybe produced by evaporatan atmosphere of hydrogen at a pressure of ten millimeters or ten centimeters whereupon the envelope 'is' exhausted and the potassium depositing the magnesium supported by the filament in ed as described; 'fBy this method nosuitable shadow offan-ob'jectin thevesseL so "as to form a windowg-canbe obtained, butfsince the vessel is dipping in the hot nitrite the temperature at that very slowly. If too immediately falls filament I-I alight ditions that glow discharge.

] tively.

It is necessary to activate such tubes and the usual method is to pass a glow discharge through the tube when co ntaining hydrogen at a pressure of of a millimeter this process being the well known onedueElster and Geitel.

A diificulty with such a process is that the hyfdrogen eats away the active surface at the point of contact with the lead, so that contact with the sensitive surface is lost.

, A method of activating such tubes whereby this disadvantage is avoided is as follows: Hydrogen isadmitted at a very low' pressure, e. g., millimeter andthe filament is caused to light. Activation then proceeds without any glow discharge. Although activation by this method will proceed without any-discharge, it is sometimes desirable to employ a potential difference of 100 volts between sensitive surface and the filament, the filament being anode. This assiststhe activation. It is thought that the activation occurs under these conditions on account of the formation of atomic-hydrogen which hydrides the surface more easily; than molecular hydrogen. Although this theory is given it is to be understood that the invention is not to be limited thereby. Surfaces produced in this way are five or six times more sensitive after activation than before. g j

':"One-example of a specific activation in accordance with-the foregoingprocess is as follows: Hydrogen is admitted to thefevacuatedvessel 5, formed with a sensitive surface 'of" sodium or potassium, through a pa-lladium valve connected to the side'tube 24 untilthe pressure is of the order or of a millimeter. (see Fig. '2.) The filament I1 contained in the" vessel is of tungsten and is heated to atemperatu'reof about 2500 K. Immediately activation begins and the process is' then continueduntil maximum sensitivity is reached. After this theactivity falls I muchhydrog'en is put in, say from .01 to .05 millimeter the activity rises but off, but on pumping out the hydrogen through the activity rises rapidly. It is possible to activate the tube with the filament alight in this way'with a voltage applied between the cathode I4 and filamentunder such conthe pressure is-insufficient to cause a Y Howeverfit'is preferable not to apply a potential.

' 1 While the inventionrelatesprimarily to the manufacture 'of' photoelectric tubes, 'applicants material into vaYphotoelect-ric tube which com prises impregnating. the inside ofsaid photoelectric tube container with alight sensitive material by electrolysis, and subsequently liberating said 'ma-terial by reverse electrolysis. i p

2. The process of'manufacturing'a 'photoelee the exhaust tube Iii-with the novel method, as defined incertain of the claims,

tric cathode which comprises immersing a glass bulb containing molten alkali metal salt in a bath of similar molten alkali metal salt, efiecting substitution of the metal ions in the glass bulb with metal ions of the enclosed salt by electrolysis, and liberating in said glass bulb such metal ions to form therein a light sensitive cathode by reverse electrolysis.

3. In the manufacture of a photoelectric tube the process comprising impregnating the inner surface of the tube with metal ions of alkali metal salt by electrolysis, evacuating the tube to a high vacuum, vaporizing magnesium from a heated filament on to the walls of the tube, applying a potential between said filament and an external bath of molten alkali metal salt to effect thereby reverse electrolysis whereby a layer of potassium is deposited on the magnesium.

4. In the manufacture of a photoelectrictube the process comprising impregnating the inner surface of a glass envelope with alkali metal ions by means of electrolysis, depositing magnesium by evaporation of the metal from an internal heated filament on to the walls of the envelope, depositing alkali metal on the magnesium by reverse electrolysis, and activating the magnesium and alkali sensitive surface by lighting said filament in the presence of a very low pressure of tric tube which comprises introducing molten alkali metal salt into a glass envelope, partially immersing said envelope in a bath of molten alkali metal salt, effecting electrolysis between the two lots of salt to cause metal ions of the salt within the envelope to enter the glass, cleaning and evacuating said envelope, vaporizing a metal to form a conductive coating upon the inner walls of said envelope, depositing alkali metal on said conductive coating by reverse electrolysis, and activating said alkali metal to produce a light sensitive surface.

7. The method of activating without glow discharge a light sensitive surface in a photoelectric tube having a cathode and a filamentary anode comprising evacuating the tube to a high vacuum, admitting to the tube a low pressure of hydrogen, and heating the filamentary anode.

8. The method of introducing light sensitive material into a photoelectric tube which comprises impregnating the inside of said photoelectric tube container with metallic potassium ions obtained by an electrolysis of molten potassium nitrite within the container, and subsequently liberating said potassium ions by a reverse electrolysis between an internal filament and an external mass of molten potassium nitrite.

9. The method of preparing an electrode for a photoelectric tube which consists in impregnating the inner surface of the tube container with metal potassium ions by electrolysis, vaporizing magnesium metal from a heated filament onto the inner surface of the tube container, and by reverse electrolysis causing the said potassium ions to deposit on the interior surface of the said magnesium metal.

ERNEST Ros'rAs. 

